In general, this invention relates to circuitry for energizing a cold cathode lamp. More particularly, it relates to a control circuit having d.c. powered digital circuits for causing the cold cathode lamp to repeatedly reignite at a pulse repetition frequency defined by a control signal output of the control circuit.
Cold cathode lamps are well known in the art. They are used in various ways including advertising signs and decorative devices. A cold cathode lamp includes an elongated tube that is formed into a desired shape to define a design. In some circumstances it is desirable to have the lamp provide steady illumination. In others, it is desirable to produce other kinds of illumination effects involving perceptible variations in light intensity produced by the lamp.
Some such lamps are mercury filled, and others are of the neon type which include a trace of argon. A characterizing electrical feature of any such cold cathode lamp is that it defines a negative resistance load after it is initially fired. That is, a relatively high voltage appears across the lamp input terminals and relatively low current flows therebetween when the lamp initially fires. Then, as additional drive current flows, the magnitude of the drive voltage decreases. Such lamps are made in various sizes and have various different ratings as to triggering voltage and sustaining voltage. The longer the lamp is made, the higher these rating voltages become, with a typical constant of proportionality for sustaining voltage being about 100 volts per foot.
A conventional arrangement for energizing a cold cathode lamp is referred to as a ballast circuit, and is designed in such a way as to be compatible with the negative resistance load presented by the cold cathode lamp. A characterizing feature of an inverter ballast for driving a cold cathode lamp is that the interconnected inverter ballast and cold cathode lamp define a self-oscillating circuit. There are significant shortcomings involved in such an overall self-oscillating circuit. Among these shortcomings is an important one relating to intensity control. As stated above, it is desirable to provide for such intensity control to make possible various types of illuminating effects. Prior attempts to meet this objective have involved amplitude control over supply voltage, which has been found to be an unsatisfactory approach.